1. Field of Invention
This invention relates to navigational systems and particularly to an autonomous navigational system for an airborne vehicle, which system selectively matches terrain features in a sensed map image with those in an associated one of prestored reference map images in order to obtain position updating information for the vehicle at each checkpoint along a preselected flight path.
2. Description of the Prior Art
Pilotless, self-guided, continuously-powered airborne vehicles, such as unmanned spacecraft and cruise missiles, can be employed to perform a wide variety of commercial and military functions. These functions may be geological, geographical, geophysical and military in nature, such as aerial photography, radar mapping, land surveys, mineral exploration, aerial reconnaisance, aerial transportation, and the destruction of a remote target with an expendable missile.
Such pilotless vehicles can be utilized to perform many of the above functions at less expense and with more safety than by using manned aircraft. This is particularly true where vehicle weight has to be minimized or where the environment might be hostile to a pilot due to rugged terrain, weather conditions and enemy-held territory.
A long-range pilotless vehicle utilizes an inertial guidance system for guiding the vehicle along its preselected flight path. An inertial guidance system may comprise three or more accelerometers mounted on gyroscope-stabilized platforms. The inherent drift in an inertial guidance system may cause the vehicle to drift a kilometer or so off course for each hour of the vehicle's flight. Weather conditions and any performance changes in the vehicle's propulsion system can further increase the vehicle drift. Guidance errors caused by changes in weather conditions or in the vehicle's propulsion system, as well as by the inherent drift in the inertial guidance system, accumulate with time. Thus, without in-flight position-correction, deviations of the vehicle from its flight path can result in a very large cumulative error in the vehicle's position over a long flight path. For example, after several hours of flight the vehicle could drift off course by, for example, ten kilometers. To compensate or correct for such errors in the position of the vehicle, the vehicle must also have the capability of periodically determining its actual in-flight position in order to update its inertial guidance system to substantially return the vehicle to its intended course.
Three different systems have been proposed for periodically determining the actual position of a long-range pilotless vehicle while it is in flight. These three proposed systems are discussed by Kosta Tsipis in his article "Cruise Missiles," Scientific American, Volume 236, Number 2, February 1977, pages 20-29. These three proposed systems are respectively called:
(1) the global-positioning satellite system,
(2) the terrain-contour-matching system, and
(3) the area-correlation system.
Essentially, each of these proposed systems computes the actual position of the vehicle in some manner and uses the difference between the vehicle's actual and intended positions to update the position coordinates of the vehicle. The velocity of the vehicle can also be changed as a function of these updated position coordinates to aid the vehicle in reaching its next checkpoint at the intended time.
The global positioning satellite system, when developed, will comprise 24 satellites in polar orbits positioned so that at least four of the satellites can be seen from any place on Earth. Passive equipment in the pilotless vehicle will compute the vehicle's actual position from satellite orbit information and the arrival times of synchronous coded signals received from the satellites that are in sight of the vehicle.
This global-positioning satellite system requires an extensive amount of equipment to be independently located at long distances from the pilotless vehicle and is essentially a time-measurement scheme for locating the actual position of the vehicle.
The terrain-contour-matching system pre-stores in a memory a set of digital maps, with each pre-stored map encompassing a different ground area along the intended flight path of the vehicle. Each large ground area is divided into smaller ground areas. Recorded in each smaller ground area of a map is a number representing the average elevation of the ground in that smaller ground area. The vehicle further includes a down-looking radar altimeter, which is capable of resolving objects on the ground that are smaller than the smaller ground areas in a pre-stored map. As the pilotless vehicle approaches ground terrain for which the memory has a map, the radar altimeter provides ground elevation data of a map area of local terrain smaller than that of the associated pre-stored map. An on-board computer compares the incoming ground elevation data with the pre-stored elevation data in the associated map to determine the actual position of the vehicle.
As described above, the terrain-contour-matching system matches the average elevations of ground in the smaller ground areas of a pre-stored map with ground elevation data provided by the radar altimeter. As a result, the larger the area encompassed by a smaller ground area, the greater the likelihood that the system will compute an erroneous actual location for the vehicle. To increase the accuracy of locating the vehicle's actual position, these smaller ground areas should be relatively small. However, if they are made too small, such that a large ground area contains a large number of small ground areas, the system becomes much more complex, heavier and more costly to implement. In addition, the vehicle may have to fly closer to the ground, or a higher resolution radar altimeter may be required, in order to produce higher resolution elevation data for matching purposes. Another disadvantage of this terrain-contour-matching system is that it will not properly operate over relatively flat terrain. As stated on page 23 of the above-identified article "the terrain-matching technique works well only over rough, hilly ground."
The area-correlation system, which is similar to the terrain-contour-matching system, periodically maps the microwave reflectivity of the local terrain that the vehicle is flying over and matches the signals from that local terrain with data from an associated one of pre-stored terrain maps to determine the vehicle's actual position. Each mapped local terrain is only part of an associated pre-stored terrain map. Page 23 of the above-cited article states that the area-correlation approach "is still in the research stage" but that "(t)his approach is possible because features such as lakes, rivers, roads, railroads and other man-made structures offer sharp `contrast edges` over a large portion of the spectrum."
None of these navigational systems are known to specifically store terrain features of terrain maps in memory, develop terrain features from the sensed local map image, and selectively compare terrain features of the sensed local and stored map images to determine actual position.